When molding polymers, the molds or tools are typically designed to allow the polymer to be demolded without being deformed. By including a draft angle in mold cavities (i.e. a taper such that the molded part is widest at the cavity opening and becomes increasingly narrow toward the deepest part of the cavity), demolding without deformation is facilitated.
In some cases, features are molded to have a shape in which a protuberance or indentation impedes withdrawal of the molded feature from the mold, and these are referred to as undercut features. Undercut features include, for instance, hook shapes because the hooks bend to the side such that the tips of the hooks are suspended to the side of the base of the hooks. Undercut features also include capped stems (e.g., stems with heads, having a nail or mushroom shape) because the caps project beyond the diameter of the base on which they rest. Undercut features further include funnel-shaped features in which the diameter of the feature gradually increases from the base to the tip. Undercut features are generally difficult to demold because the overhanging portion of the feature cannot be usually pulled from the mold cavity without breaking or permanently deforming the feature.
Multiple techniques have been developed to overcome the complications of demolding undercut features. In some cases, the mold is made of multiple parts that can be partially disassembled after molding, thus opening each of the cavities and allowing the feature to be easily removed. Such an approach can be effective; however, it typically requires complicated tooling. Alternatively, capped stems can be produced by first molding straight stems with no undercut, and then capping those stems in a separate shaping step after demolding. Yet another general approach involves allowing undercut features to bend during demolding, and then using the resilience of the polymer to bring it back to the as-molded shape, for example using hook shapes. Hook shapes formed by such methods include either a constant or decreasing cross-sectional area moving from the base of the hook to the tip of the hook. The shape of a hook allows the hook to be demolded by bending and/or maneuvering the hook to come out of the cavity, so shear strains and compressive strains are minimized. One disadvantage to employing a hook shape as an undercut feature is that a hook usually bends or breaks under a lower force than other undercut features, such as capped stems or funnels.